In the field of radio communication, there has been actively employed in recent years MIMO (multiple-input and multiple-output) technology, in which speed and quality of signal transmission are improved by carrying out transmission/reception using a plurality of antennas on the transmitter side and receiver side. MIMO technology includes SU-MIMO (single user MIMO) for a single user (receiver), and MU-MIMO (multiple user MIMO) for a plurality of users (receivers).
Further, there is known in the art (for example, Patent Document 1) a massive-MIMO transmission scheme in which there is used a large number of antenna elements so as to achieve a further increase in speed and a reduction in interference in signal transmission, (for example, 100 elements or more) in a high-frequency band (for example 10 GHz or more), and use of which also enables miniaturization of antennas and a wide bandwidth to be attained.
With massive-MIMO, there can be achieved advanced beamforming (BF) by use of a large number of antenna elements, compared to conventional MIMOs. Beamforming is a technology for controlling a directivity and/or shape of beams (transmission beams corresponding to transmission antennas, or reception beams corresponding to reception antennas) by use of a plurality of antenna elements. With MIMO, phase and amplitude can be controlled for each antenna element, thus the larger the number of antenna elements used, the greater the degree of freedom of beam control.
As one mode of beamforming, fixed beamforming is exemplified. In fixed beamforming, a beamforming weight (fixed beam) to be used is selected from among a plurality of beamforming weights prepared in advance. In fixed beamforming, beamforming that controls fixed beams and coding that achieves compensation for multiplexing between a plurality of streams (transmission-side precoding and reception-side postcoding) are carried out separately. For coding that achieves compensation for multiplexing between a plurality of streams, it is necessary for a user equipment to feedback various information to a base station.
However, with MU-MIMO, as the number of user equipments that performs communication simultaneously increases, an amount of feedback information also increases. Patent document 2 discloses a feature in which the RSSI of a signal from a base station is measured in a user equipment, and if it is determined that the RSSI is a threshold value or higher, the channel state is estimated and the channel state information is fed back to the base station, and if it is determined that the RSSI value is lower than the threshold value, channel state information is not fed back to the base station. According to this feature, the amount of information fed back from the user equipment to the base station can be reduced.